1. Field of the Invention
The present invention relates to novel water-soluble (meth)acrylic acid/methallylsulfonate copolymers, and to the use thereof as scale inhibitors for aqueous environments.
2. Description of the Prior Art
It is known to this art that the breakdown of the calcium carbonate equilibrium in water by heating, degassing or increasing the pH thereof gives rise to the formation of encrusting deposits of alkaline earth metal carbonates on the walls of water-confining vessels, or in the piping comprising the various cooling circuits or boilers. As the thermal conductivity of these deposits is incomparably lower than that of metals, there is a marked loss in the heat exchange capacities of such systems with concomitant loss or waste of energy. It is, therefore, obviously necessary to prevent the formation of the aforesaid encrusting deposits, commonly referred to as scale.
And to date numerous means have been proposed for preventing scaling, for example:
(i) Partial or total removal of the calcium and magnesium in the water, before use, by means of ion exchange or decarbonation with lime; or
(ii) Addition of an acid agent to the water in the circuit; or also
(iii) Addition of agents which sequester the calcium and magnesium ions.
However, these processes are expensive and can also themselves present disadvantages, such as corrosion. Therefore, preference is frequently given to a more economical and well-known process which consists in adding water-soluble chemical additives, in proportions which are typically between 0.2 and several tens of mg/liter, to the hard water; these additives which are more commonly referred to as scale inhibitors, exert their influence on the formation of crystals. They disturb or disrupt their growth such that precipitation is retarded and the encrustation, if any indeed be produced, is rendered brittle.
Stated differently, by adding homeopathic amounts of chemical additives to the water, the scaling of the heat exchangers is retarded.
Some of these adjuvants, such as (1) the alkali metal polyphosphates, have been known for such purpose for a very long time; but same exhibit the disadvantage of hydrolyzing to orthophosphates, whereupon their scale-inhibiting attributes vanish, and precipitation of alkaline earth metal orthophosphates also takes place, which leads to a result diametrically opposed to that desired.
Other additives have also been similarly employed, such as:
(2) Phosphonic and polyphosphonic acids, which may or may not contain nitrogen, and their alkali metal salts;
(3) Low molecular weight homopolymers of acrylic acid or methacrylic acid and the corresponding alkali metal salts thereof;
(4) Low molecular weight copolymers of acrylic acid and methacrylic acid and the corresponding alkali metal salts thereof;
(5) Very low molecular weight homopolymers of maleic acid and its salts, and copolymers thereof; and
(6) 1000 to 25,000 molecular weight water-soluble copolymers of a monovinyl comonomer, e.g., (meth)acrylic acid, and a major amount of a vinyl sulfonate comonomer (as "below 25 mol percent the copolymer is no longer suitably effective in acid conditions of pH 3 or in the presence of useful amounts of zinc"), or salt form thereof; see U.S. Pat. No. 3,706,717.
Moreover, it too will be appreciated that the aforesaid list is by no means exhaustive.
It has also been determined, in particular, that polymers of ethylenic diacids, such as maleic acid, are better inhibitors of the precipitation of calcium and magnesium scales than are the homopolymers or copolymers of ethylenic monoacids, such as acrylic or methacrylic acids.
Admittedly, the scale-inhibiting attributes of polymers of ethylenic monoacids are quite good, as are those of the alkali metal polyphosphates. However, when the temperature of the water and also the residence or dwell time of the water in the particular circuit increase greatly, alkali metal polyphosphates lose much of their scale-inhibiting efficacy and it has been ascertained that polymers of ethylenic monoacids become poorer scale inhibitors than polymers of ethylenic diacids.
And whatever the procedure employed for the polymerization of ethylenic monoacids, whatever the transfer agents used and whatever the catalyst selected for the formation of free radicals, polymers are obtained, the scale-inhibiting properties of which are essentially equivalent in each case, but are demonstrably poorer than those of very low molecular weight polymaleic acid.
Thus, by way of example, the scale-inhibiting effectiveness of polymers prepared from ethylenic monoacids is good at moderate temperatures and up to 95.degree. C., but upon reaching the boiling point of the water, and even 101.5.degree. C. in the case of sea water, such scale-inhibiting effectiveness is then poorer than with polymaleic acid.
This rule applies whatever the polymerization system used, namely, persulfate associated with acetic acid, hydrogen peroxide associated with acetic acid or with isopropyl alcohol in larger or smaller amounts, or hydrogen peroxide associated with hydroxylamine sulfate and with isopropyl alcohol. Moreover, in the case of methacrylic acid homopolymer, the viscosities of the resultant products are very high and same are totally inadequate as regards providing good scale-inhibiting characteristics.